Relative permeabilities in immiscible two-phase (wetting and non-wetting phase) flow through porous media were estimated theoretically by using a network model. In the model, a porous material is assumed to be a network which is composed of a large number of pore segments of a constant length. The pore diameter distribution is also assumed to be the Rosin-Rammler. The flow rate through the network was calculated with the Kirchhoff law under a certain pressure difference between the both sides of the network. In the case of relative permeability estimation of non-wetting phase, it is assumed that the wetting liquid occupies smaller pores or smaller flow paths than does the non-wetting liquid. On the other hand, in the case of wetting phase it is assumed that the non-wetting liquid occupies some of larger pores than the diameter Dmin. The model works best for estimation of relative permeabilities of wetting and non-wetting liquids, and the relations between saturation and relative permeability agree well with the experimental findings of Wyckoff. There is no effect of the value of Dmin on the relation between saturation and relative permeability estimated from the model.